The purpose of this study is to determine whether in vivo administration of interleukin-12 (IL-12) will shift cytokine expression during T-independent immune responses and enhance the effectiveness of vaccination against polysaccharide antigens. IL-12 may be especially beneficial for enhancing the usefulness of pneumococcal and meningococcal polysaccharide vaccines, since it stimulates production of interferon-gamma and increases secretion of antibody isotypes that are efficient in mediating anti-bacterial immunity. For the experiments in this study, BALB/c and C57BL/6 mice will be inoculated with IL-12 and polysaccharide vaccine, and levels of induced cytokines and antibodies will be monitored at various times thereafter. The ability of IL-12 to cause shifts in cytokine RNA expression will be examined by RT-PCR and ribonuclease protection assays. The levels and specificities of induced antibodies will be quantitated using isotype-specific ELISAs. Intermediary cytokines potentially responsible for the observed effects will be identified using mice with targeted disruptions in cytokine genes and the role of endogenous IL-12 in regulating responsiveness will be examined using IL-12 knockout mice. Immunodeficient mice lacking specific lymphoid cell subsets will be exploited to determine whether the effect of IL-12 are due to activated NK cells or to direct stimulation of B-cells through binding to the newly described B-cell IL-12 receptor. The biological functions of the induced antibodies will be determined by complement fixation and opsonization assays against the target bacterial strains. Finally, the ability of IL-12 to enhance in vivo protection against lethal pneumococcal and meningococcal infections will be tested by direct inoculation of virulent bacterial strains into vaccinated mice. Infections due to S. pneumonia and N. meningitidis are the leading causes of pneumonia, meningitis, and otitis media, causing an estimated 7.5 million cases/year in the U.S. and over 100 million cases/year worldwide. Current vaccines are of only limited effectiveness. Thus, the use of IL-12 as a vaccine adjuvant may provide a novel approach for protection against these diseases and could have immediate impact on human health.